Weil-McLain WMB-155c Combi unit trouble

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  • Weil-McLain WMB-155c Combi unit trouble

    Thomas, need some help! Just replaced an old combi unit with a new WMB-155c, have it plumbed and all the electronics connected per the manual (which can be found here: https://www.weil-mclain.com/sites/de...%20Version.pdf

    The issue is that when starting the unit it goes through the "FH" combustion chamber purge and starts to rythmatically increase the system psi from around 20 to nearly 30. The jumps in pressure correspond to the activation of the integrated circulator pump which is also rythmatically activating and deactivating in time with the pressure jumps. It does this for a minute or two then shuts off flashing the error code "A03" which is: "High limit protection: pump does not run or no water circulation" (pg 88 of the manual). This certainly seems like an issue with air in the system, but having purged the air from all the places that are apparent in the system (on the two zones, above the expansion tank, from the automatic air purge in the boiler's housing) with zero effect on the pressure anomaly on start up, I can't for the life of me figure out what's going on!

  • #2
    Hi
    I am not familiar with this type of boiler.
    A few things come to mind, Is the expansion tank new also? it may be defective, is the expansion tank the right size for this system?
    Are all of the valves on the heating loops full open?
    Are the Zone valves open?
    Are you using Taco zone valves? Taco zone valves take up to 2 minutes to open, although the end switch would not engage until it is fully open, if the boiler is wired for a hot boiler, it may start pumping before the zone valve is open.
    It sounds like back pressure to me, like the loop is not clear for the water to move.
    That's the best I can do with this.
    I answer all plumbing and heating questions, all day long.

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